Acting President Oleksandr Turchynov told the parliament in Kyiv that rebels used a portable air defence missile to bring down the helicopter. He said 14 died, including Gen. Serhiy Kulchytskiy, according to the Interfax news agency, which earlier gave the wrong first name for the general.

Ukraine’s National Guards put the death toll at 12, including Kulchytskiy, and said that one soldier was badly wounded, but added that the information is still being clarified.

Slovyansk, a city of 120,000 residents 100 miles (160 kilometres) from the Russian border, has become the epicenter of fighting between pro-Russia insurgents and government forces in recent weeks. Its residential areas have regularly come under mortar shelling from government forces, causing civilian casualties and prompting some residents to flee.

An Associated Press reporter saw the helicopter go down amid a trail of black smoke. Gunshots were heard in Slovyansk near the crash site and a Ukrainian air force jet was seen circling above. It was too dangerous to visit the site itself.

Turchynov said the helicopter was flying troops to a hill outside Slovyansk where Ukrainian forces have set up positions.

Interfax said Kulchytskiy had once served in the Soviet army and was in charge of combat training for Ukraine’s National Guards.

Slovyansk is in the Donetsk region, one of the two provinces in eastern Ukraine that have declared independence from the government in Kyiv.

The Kyiv government condemns the insurgency roiling the east as the work of “terrorists” bent on destroying the country and accuses Russia of fomenting it. Russia denies the accusations, saying it has no influence over rebels, who insist they are only protecting the interests of Russian-speakers in the east.

Still, fighters from Russia, including from the battle-hardened region of Chechnya, have been appearing recently in the ranks of the separatists.

An insurgent in Donetsk, who identified himself only by his nom de guerre, Baran (Ram), told reporters Thursday that 33 Russian citizens were among the rebels killed in the city in fighting earlier this week.

Dozens of men were killed – some insurgent leaders said the death toll might be up to 100 fighters – when Ukrainian forces used combat jets and helicopter gunships Monday to dislodge the rebels who tried to take control of the city airport.

Also Thursday, an insurgent leader confirmed that his fighters were holding four missing observers from the Organization for Security and Cooperation in Europe and promised they would be released shortly. Vyacheslav Ponomarev, the self-proclaimed “people’s mayor” of Slovyansk, told the AP that the monitors – who are from Turkey, Switzerland, Estoniaand Denmark – were safe.

“I addressed the OSCE mission to warn them that their people should not over the coming week travel in areas under our control. And they decided to show up anyway,” Ponomarev said.

“We will deal with this and then release them,” he said, without setting a specific timeframe.

The OSCE had lost contact with the team in Donetsk on Monday evening. Their teams have been deployed to Ukraine to monitor security situation following Russia’s annexation of Crimea and the rise of the pro-Russia separatist insurgency in eastern Ukraine. They also observed Sunday’s presidential vote, won by billionaire candy magnate Petro Poroshenko.

Poroshenko has promised to negotiate with people in the east but also vowed to uproot the armed rebels.

The mood in Donetsk was calm Thursday, although many businesses have stopped opening due to fears of renewed fighting following Monday’s ferocious battle.

The separatists in Ukraine have pleaded to join Russia, but President Vladimir Putin has ignored their appeal in an apparent bid to de-escalate tensions with the West and avoid a new round of Western sanctions.

Putin has supported an OSCE peace plan that calls for ending hostilities and launching a political dialogue, and has said Russia would work with Poroshenko. But Russia has repeatedly urged the Ukrainian government to end its military operation against the separatists.

Russian Foreign Minister Sergey Lavrov on Thursday called for quick international mediation to persuade Kyiv to halt what he described as a “punitive operation” in the east.

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Leonard reported from Donetsk, Ukraine. Laura Mills in Kyiv and Vladimir Isachenkov in Moscow contributed to this report.